Scanning circuit for image pickup apparatus

ABSTRACT

This invention provides an image pickup device comprising a plurality of pixels each including a photoelectric conversion unit, a semiconductor area to which a signal from the photoelectric conversion unit is transferred, a transfer switch for transferring the signal from the photoelectric conversion unit to the semiconductor area, and a read unit for reading out the signal from the semiconductor area, and a drive circuit for outputting a first level at which the transfer switch is set in an OFF state, a second level at which the transfer switch is set in an ON state, and a third level between the first level and the second level, wherein the drive circuit controls to hold the third level for a predetermined time while the transfer switch is changing from the ON state to the OFF state.

This application is a divisional of U.S. application Ser. No. 12/348,996filed Jan. 6, 2009, now allowed, which is a divisional of applicationSer. No. 09/929,037, filed Aug. 25, 2001, now U.S. Pat. No. 7,688,371,the content of each of which are incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image pickup apparatus for pickingup an object image and a drive method therefor.

2. Related Background Art

As typical solid-state image pickup element, an element formed from aphotodiode and CCD shift register and an element called an APS (ActivePixel Sensor) formed from a photodiode and MOS transistor are known.

An APS includes for each pixel, a photodiode, MOS switch, amplificationcircuit for amplifying a signal from the photodiode, and has maytechnological advantages that XY addressing is possible, and a sensorand signal processing circuit can be formed on one chip, for example.However, since many elements are included in one pixel, the chip sizethat determines the scale of the optical system can hardly be reduced,so CCDs are currently most popular. An APS has received a great deal ofattention because the MOS transistor micropatterning technique isimproved, and strong demands have arisen to “make a sensor and signalcircuit on one chip” or to “reduce power consumption”.

FIG. 13 is an equivalent circuit diagram of an APS. Referring to FIG.13, a transfer switch Q1 transfers photocharges from a photodiode 101 toa floating diffusion area (FD). A reset switch Q2 resets the floatingdiffusion area. An input MOS transistor Q3 is included in a sourcefollower for outputting the voltage in the floating diffusion area. Aselection switch Q4 selects a pixel. The APS also includes a powersupply line 102, reset switch control line 103, transfer switch controlline 104, selection switch control line 105, and signal output line 106.The operation of the APS using a scheme, in which the selection switchcontrol line 105 for controlling the selection switch Q4 is commonlyarranged in the row direction so as to select a row and transfer chargesof one row to a line memory at a time, will be described.

FIG. 14 shows a drive pulse used to read out a pixel (row).

Before the start of accumulation operation, while keeping the resetswitch Q2 turned on, the transfer switch Q1 is turned on to temporarilyreset the photodiode 101, as shown in FIG. 14. The floating diffusionarea is set in the floating state, and the selection switch forreading-out is turned on. A voltage corresponding to the voltage in thefloating diffusion area is output to the signal output line 106 by asource follower formed from a load connected to the input MOS transistorQ3 and signal output line 106. This output is sampled into a memory.That is, reset noise is sampled. After that, to transfer thephoto-signal charges to the floating diffusion area (FD), the transferswitch is turned on and off. Transfer is done while keeping the floatingdiffusion area in the floating state. For this reason, the voltage inthe floating diffusion area is a voltage lower than a voltage Vresimmediately after the reset, by Q/CFD (Q is transferred charges, and CFDis the capacitance of the FD portion), which is given byVFD=Vres−Q/CFDThat is, Q/CFD is superposed on the reset voltage containing differentreset noise components at each reset. Since a signal corresponding tothe voltage is output to the signal output line 106, this signal issampled (“photo-signal plus sampling of reset noise” in FIG. 14).

Finally, the “reset noise” signal and “photo-signal+reset noise” signalare subtracted from the photo-signal by a differentiating circuitwhereby the reset noise which is reset to different voltages at eachreset can be removed.

Especially, when a buried photodiode is used as the photodiode 101 inFIG. 13, the reset noise can be completely removed, and a high S/N ratiocan be obtained.

However, recent APSs have problems of a residual image and random noise.The reasons for this will be described below in detail.

As described above, recent APSs use micropatterned MOS transistors,thereby realizing a small pixel size. For this reason, a residual imageor random noise due to new reasons based on the micropatterned MOStransistors occurs.

FIG. 15 is a sectional view of an APS using a micropatterned MOStransistor. FIG. 16 is a potential chart corresponding to the sectionalview of FIG. 16. As shown in FIG. 15, a surface p-type area 304 isformed on an n-type area 305 of a photodiode (called a buriedphotodiode). Charges accumulated in the n-type area 305 are completelytransferred to a floating diffusion area 303 through a transfer switch(SW) 302 by a depletion layer extending from a PWL (p-well) 301 andsurface p-type area 304. The floating diffusion area 303 is connected toa reset electrode 307 through a reset switch (SW) and can be set to apredetermined reset potential.

In the APS using a micropatterned MOS transistor, a potential pocket andbarrier are formed, as indicated by A and B in FIG. 16. As a result,some of charges in the transfer switch area return to the photodiode togenerate a residual image or random noise.

The pocket (A in FIG. 16) and barrier (B in FIG. 16) of the potentialwill be described in more detail.

To form a micropatterned MOS transistor, annealing in the manufacturingprocess must be executed at a lower temperature in a shorter time.Hence, an impurity, such as boron or phosphorus, for determining theconductivity type is diffused not only simply by heat but also throughdefects and the like, i.e., the influence of transient enhanceddiffusion phenomenon becomes conspicuous. As a result, boron fordetermining the p-type well region segregates to an area correspondingto B in FIG. 16, thereby forming a potential barrier. This phenomenon isknown as a reverse short channel effect in a micropatterned MOStransistor.

Such a phenomenon typically occurs in an area having a channel length Lof 1 μm or less and, more particularly, a channel length L equal to orsmaller than 0.7 μm.

On the other hand, an area corresponding to A in FIG. 16, i.e., aportion where the n-type area 305 of the photodiode 101 connects to thetransfer switch 302 determines the signal charge transfercharacteristic. The positional relationship between the surface p-typearea 304 and the n-type area 305 of the photodiode is very important.Especially when a micropatterned MOS transistor is used, the voltage tobe applied to the gate of the transfer switch 302 must be reduced. Inthis case, however, the potential of the transfer switch cannot besufficiently dropped, and the charges are difficult to be transferred.In some cases, to promote charge transfer, an n-type bypass area 308 ispositively formed, as shown in FIG. 15.

The width of such a bypass area 308 is 0 to 0.5 μm. If the bypass area308 is too narrow, transfer is difficult. If the transfer width is toolarge, a potential pocket is formed. When the voltage applied to thegate of the transfer switch 302 is high, the voltage can compensate fortransfer, and be designed as a value that prevents any potential pocket.However, when a micropatterned MOS transistor is used, the voltagecannot compensate for transfer, and the width controllability is asstrict as 0.05 μm or less, i.e., a stricter controllability than thatfor the gate length of a micropatterned MOS transistor is required. As aconsequence, a potential pocket is readily formed.

In the conventional art, charges remain under the gate of the transferswitch 302 due to the above-described potential pocket (A in FIG. 16) orpotential barrier (B in FIG. 16).

The remaining amount is given byRemaining amount∝(VTXH−VTXth−VFD)where VTHX is the high-level gate voltage of the transfer switch 302,VTXth is the threshold voltage of the transfer switch 302, and VFD isthe voltage in the floating diffusion area.

When some or all of the residual charges return to the photodiode, aresidual image is generated. Depending on the operation condition,residual charges are generated even in a dark state and return to thephotodiode. The residual charges thermally fluctuate to make randomnoise.

FIGS. 17A to 17D show potential states when the transfer switch isturned on and off. FIG. 17A is a potential chart showing a stateimmediately after the floating diffusion area is reset. Signal chargesare accumulated in the photodiode. FIG. 17B is a potential chart showinga state wherein the transfer switch is turned on to transfer the signalcharges to the floating diffusion area. At this time, the potential ofthe floating diffusion area is increased by the signal charges.

Since the surface potential under the gate of the transfer switchbecomes lower than the potential in the floating diffusion areadepending on the amount of signal charges, charges are generated evenunder the gate of the transfer switch. FIG. 17C shows a state whereinthe transfer switch is turned off, i.e., a state wherein charges inducedunder the gate of the transfer switch cannot completely move to thefloating diffusion area. The charges that cannot completely move returnto the photodiode side to cause a residual image or random noise.

In a CCD, the power supply voltage is high. For this reason, even whenmaximum signal charges are transferred, the surface potential of thetransfer switch is always higher than the vertical CCD potential, and nocharges are generated under the gate of the transfer switch.

In the APS using a micropatterned MOS transistor, the power supplyvoltage is low. For this reason, when at least maximum signal chargesare transferred, the state shown in FIG. 17B occurs, and a potentialrelationship as in a CCD can hardly be formed. In addition, when thereset voltage is low, the state shown in FIG. 17B occurs even in a darkstate, resulting in random noise in the dark state.

SUMMARY OF THE INVENTION

It is an object of the present invention to obtain an image almost freefrom noise.

In order to achieve the above object, according to an aspect of thepresent invention, there is provided an image pickup apparatuscomprising a plurality of pixels each including a photoelectricconversion unit, a semiconductor area to which a signal from thephotoelectric conversion unit is transferred, a transfer switch adaptedto transfer the signal from the photoelectric conversion unit to thesemiconductor area, and a read unit adapted to read out the signal fromthe semiconductor area, and a drive circuit adapted to output a firstlevel at which the transfer switch is set in an OFF state, a secondlevel at which the transfer switch is set in an ON state, and a thirdlevel between the first level and the second level, wherein the drivecircuit controls to hold the third level for a predetermined time whilethe transfer switch is changing from the ON state to the OFF state.

According to another aspect, there is provided an image pickup apparatuscomprising a plurality of pixels each including a photoelectricconversion unit, a semiconductor area to which a signal from thephotoelectric conversion unit is transferred, a transfer switch adaptedto transfer the signal from the photoelectric conversion unit to thesemiconductor area, and a read unit adapted to read out the signal fromthe semiconductor area, and a drive circuit adapted to output a signalfor controlling the transfer switch so that a time during which thetransfer switch changes from an ON state to an OFF state becomes longerthan a time during which the transfer switch changes from the OFF stateto the ON state.

According to still another aspect, there is provided an image pickupapparatus comprising a plurality of pixels each including aphotoelectric conversion unit, a semiconductor area to which a signalfrom the photoelectric conversion unit is transferred, a transfer switchadapted to transfer the signal from the photoelectric conversion unit tothe semiconductor area, and a read unit adapted to read out the signalfrom the semiconductor area, and a drive circuit adapted to control thetransfer switch, wherein a substantial driving force of the drivecircuit for changing the transfer switch from an OFF state to an ONstate is higher than a substantial driving force for changing thetransfer switch from the ON state to the OFF state.

According to still another aspect, there is provided an image pickupapparatus comprising a plurality of pixels each including aphotoelectric conversion unit, a semiconductor area to which a signalfrom the photoelectric conversion unit is transferred, a transfer switchadapted to transfer the signal from the photoelectric conversion unit tothe semiconductor area, and a read unit adapted to read out the signalfrom the semiconductor area, and a drive circuit adapted to control thetransfer switch, wherein the transfer switch comprises a transistor of afirst conductivity type, and the drive circuit includes at least astructure formed by connecting the transistors of the first conductivitytype in series.

According to still another aspect, there is provided an image pickupapparatus comprising a plurality of pixels each including aphotoelectric conversion unit, a semiconductor area to which a signalfrom the photoelectric conversion unit is transferred, a transfer switchadapted to transfer the signal from the photoelectric conversion unit tothe semiconductor area, and a read unit for reading out the signal fromthe semiconductor area, and a drive circuit adapted to output a signalfor controlling the transfer switch so that a fall speed Voff forchanging the transfer switch from an ON state to an OFF state has arelation 10 V/μsec>Voff.

According to still another aspect, there is provided an image pickupapparatus comprising a plurality of pixels each including aphotoelectric conversion unit, a semiconductor area to which a signalfrom the photoelectric conversion unit is transferred, a transfer switchadapted to transfer the signal from the photoelectric conversion unit tothe semiconductor area, and a read unit adapted to read out the signalfrom the semiconductor area, and a drive circuit adapted to control thetransfer switch, wherein the drive circuit includes a constant currentcircuit.

According to still another aspect, there is provided a drive method foran image pickup apparatus including a plurality of pixels each includinga photoelectric conversion unit, a semiconductor area to which a signalfrom the photoelectric conversion unit is transferred, a transfer switchadapted to transfer the signal from the photoelectric conversion unit tothe semiconductor area, and a read unit adapted to read out the signalfrom the semiconductor area, comprising:

an output step of outputting a first level at which the transfer switchis set in an OFF state, a second level at which the transfer switch isset in an ON state, and a third level between the first level and thesecond level, wherein the third level is held for a predetermined timewhile the transfer switch is changing from the ON state to the OFFstate.

According to still another aspect, there is provided a drive method foran image pickup apparatus including a plurality of pixels each includinga photoelectric conversion unit, a semiconductor area to which a signalfrom the photoelectric conversion unit is transferred, a transfer switchadapted to transfer the signal from the photoelectric conversion unit tothe semiconductor area, and a read unit adapted to read out the signalfrom the semiconductor area, comprising:

an output step of outputting a signal for controlling the transferswitch so that a time during which the transfer switch changes from anON state to an OFF state becomes longer than a time during which thetransfer switch changes from the OFF state to the ON state.

According to still another aspect, there is provided a drive method foran image pickup apparatus including a plurality of pixels each includinga photoelectric conversion unit, a semiconductor area to which a signalfrom the photoelectric conversion unit is transferred, a transfer switchadapted to transfer the signal from the photoelectric conversion unit tothe semiconductor area, and a read unit adapted to read out the signalfrom the semiconductor area, comprising:

an output step of outputting a signal for controlling the transferswitch so that a fall speed Voff for changing the transfer switch froman ON state to an OFF state has a relation 10 V/sec>Voff.

The above and other objects, features, and advantages of the presentinvention will become apparent from the following detailed descriptionin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an equivalent circuit diagram showing the periphery of thegate drive circuit of a transfer switch used for an embodiment of thepresent invention;

FIG. 2 is a circuit diagram of the gate drive circuit of a transferswitch used for the first embodiment;

FIG. 3 is a circuit diagram of the gate drive circuit of a transferswitch used for the second embodiment;

FIG. 4 is a circuit diagram of the gate drive circuit of a transferswitch used for the third embodiment;

FIG. 5 is a circuit diagram of the gate drive circuit of a transferswitch used for the fourth embodiment;

FIG. 6 is a view showing a one-chip solid-state image pickup apparatusaccording to the fifth embodiment;

FIG. 7 is a view showing a one-chip solid-state image pickup apparatusaccording to the sixth embodiment;

FIG. 8 is a graph showing the relationship between the fall speed and aresidual image;

FIG. 9 is a waveform chart of an input to the gate of the transferswitch;

FIG. 10 is a waveform chart of an input to the gate of the transferswitch;

FIG. 11 is a waveform chart of an input to the gate of the transferswitch;

FIG. 12 is a block diagram showing a digital still camera;

FIG. 13 is an equivalent circuit diagram of a typical pixel of an APS;

FIG. 14 is a drive timing chart of the APS;

FIG. 15 is a sectional view of a photodiode, transfer switch, andfloating diffusion area when a micropatterned MOS transistor is used;

FIG. 16 is a potential chart corresponding to FIG. 13; and

FIGS. 17A, 17B, 17C and 17D are potential charts for explainingconventional problems and the principle of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The outlines of the embodiments of the present invention will bedescribed below. The present inventors examined and found the“relationship between the fall speed and a residual image” shown in FIG.8. FIG. 9 shows an example of a pulse waveform of a transfer switch. Thefall speed means the speed of voltage drop during the trailing edgeperiod shown in FIG. 9.

As shown in FIG. 8, as the fall speed becomes low, the residual image isreduced. When the fall speed is low, the time while the potentialstructure shown in FIG. 17D is maintained can be prolonged, and as aconsequence, the residual charges can be removed to the floatingdiffusion area.

When the fall speed is set to 500 V/μsec, a residual image of about 1%is generated. Noise level allowable as an image is generally about −46to −48 dB. When the fall speed is set to 500 V/μsec, that level cannotbe satisfied. When the fall speed is set to 10 V/μsec, theabove-described level can be satisfied, and a satisfactory image can beobtained.

The characteristic features of the potential structure shown in FIG. 17Dare that (1) the surface potential under the gate of the transfer switchis higher than the Fermi level of carriers in the floating diffusionarea, and (2) the potential barrier at the portion B is eliminated bythe influence of the drain field.

When the time while such a potential structure is maintained isprolonged, most residual charges under the gate of the transfer switchcan be removed to the floating diffusion area.

It is important to hold the potential structure shown in FIG. 17D for apredetermined time. For example, a ternary pulse which temporarily holdsthe voltage for generating the potential structure shown in FIG. 17D andthen is turned off may be used, as shown in FIG. 10. The pulse is notlimited to a ternary pulse, and any other multilevel pulse may be used.A waveform that exhibits curved level attenuation as shown in FIG. 11may also be used.

The embodiments of the present invention will be described below indetail with reference to the accompanying drawings.

The first embodiment of the present invention will be described.

FIG. 1 is a circuit diagram schematically showing the periphery of thedrive circuit of a transfer switch when a plurality of pixels shown inFIG. 12 are arrayed. The arrangement shown in FIG. 1 includes a pixel1001 which has the same structure as that shown in FIG. 13, a gate drivecircuit 1002 for operating a transfer switch Q1 in the pixel 1001, and ascanning circuit 1003 for controlling the transfer switches on each rowbasis.

In this embodiment, a waveform shown in FIG. 10 is formed using a gatedrive circuit shown in FIG. 2. Referring to FIG. 2, to output a lowlevel, middle level, and high level, the circuit is designed to input(1,0,0), (0,1,0), and (0,0,1) from the scanning circuit 1003 to inputterminals 1105, 1106, and 1107 of the scanning circuit, respectively. Inthis case, “1” is HIGH level, and “0” is LOW level. The input terminals1105, 1106, and 1107 of the scanning circuit are connected to thecontrol terminals of switches. A low-level signal, middle-level signal,and high-level signal are output from a low-level supply line 1102,middle-level supply line 1103, and high-level supply line 1104 to anoutput terminal 1101 through the switches, respectively. The outputterminal 1101 is connected to the gate of the transfer switch Q1 of thepixel 1001 shown in FIG. 1.

More specifically, the high level of the transfer switch is set to 5.0V, the middle level to 3.0 V, and the low level to 0.0 V. The middlelevel holding time is 0.5 μsec. As a result, the residual image isreduced to 0.1% or less. In addition, random noise is suppressed.Particularly in the dark state, no random noise due to charge transferis observed. As a result of this embodiment, the residual image isimproved more than in the graph “fall speed vs. residual image” shown inFIG. 8. The probable reason for this is that the potential structureshown in FIG. 17D can be held for a longer time, as compared to thewaveform shown in FIG. 9.

FIG. 3 shows the gate drive circuit of a transfer switch according tothe second embodiment of the present invention. An input terminal from ascanning circuit 1003 shown in FIG. 1 is represented by 1206 here.Referring to FIG. 3, the input terminal 1206 of the scanning circuit1003 shown in FIG. 1 is connected to the gate of a PMOS transistor 1201and the gate of an NMOS transistor 1202. The drain of the PMOStransistor 1201 and the drain of the NMOS transistor 1202 are commonlyconnected to a capacitor 1204 and the noninverting input terminal (+) ofa voltage follower amplifier 1205. The source of the NMOS transistor1202 is connected to a constant current source 1203. An output terminal1207 is connected to the gate of a transfer switch Q1 of a pixel 1001shown in FIG. 1.

The fall speed of the gate of the transfer switch Q1 is determined bythe capacitance value of the capacitance 1204 and the current value ofthe constant current source 1203.

As a result, a trapezoidal waveform as shown in FIG. 9 is formed. Inthis embodiment, the fall speed is set to 10 V/μsec. Consequently, theresidual image can be suppressed to 0.4%, as shown in FIG. 8. Inaddition, random noise due to charge transfer particularly in the darkstate is not observed.

In the above-described first embodiment, the optimum gate voltage forforming the potential structure shown in FIG. 17D is 3.0 V. This voltagehowever changes due to the depletion voltage of the photodiode. That is,the voltage varies due to manufacturing variations, and consequently,the yield of samples capable of improving the residual image or randomnoise may decrease.

However, when a trapezoidal wave as in this embodiment is used, apredetermined time is actually held at any gate voltage. For thisreason, even when the optimum gate voltage varies, the residual imageand random noise characteristics are improved.

FIG. 4 shows the gate drive circuit of a transfer switch according tothe third embodiment of the present invention. An input terminal from ascanning circuit 1003 shown in FIG. 1 is represented by 1301 here. AsMOS transistors, NMOS transistors are used.

Referring to FIG. 4, the input terminal 1301 of the scanning circuit1003 shown in FIG. 1 is connected to MOS transistors Q12 and Q15 throughan inverter. Between the power supply line and the GND line, MOStransistors Q12, Q13, and Q14 are connected in series, and MOStransistors Q11, Q16, Q21, and Q31 are connected in series. The MOStransistor Q15 is connected in parallel between the source and the drainof the MOS transistor Q14. The drain of the MOS transistor Q14(connection point between the MOS transistor Q13 and the MOS transistorQ14) is commonly connected to the gates of the MOS transistors Q11 andQ16. An output terminal 1302 as the drain connection point between theMOS transistor Q11 and the MOS transistor Q16 is connected to the gateof a transfer switch Q1 of a pixel 1001 in FIG. 1.

In this embodiment, the device is designed such that the output from thescanning circuit for row selection goes high in the selection mode, andthe common control line of the gate of the transfer switch goes low whenturning on the transfer switch. As the transfer switch, an NMOStransistor is used.

This embodiment has the following characteristic features.

(1) The output unit is constructed by connecting NMOS transistors inseries.

(2) The fall speed of the transfer switch is determined by thecapacitance of the gate line of the transfer switch and the ONresistances of the MOS transistors Q21 and Q31.

The device is designed to have a fall speed of 10 V/μsec. To design asmall current value by considering only the size of each NMOStransistor, a large space is required. However, with the arrangement ofthis embodiment, the drive circuit can be formed without largelychanging the size. Since the blanking period during which charges aretransferred from the photodiode has a temporal limitation, the risespeed in turning on the transfer switch must be high, as in the priorart. Hence, a leading edge time Ton and trailing edge time Toff has arelation Ton<Toff.

In this embodiment, the driving force in changing the control line ofthe transfer switch from the OFF state to the ON state is set to beactually higher than that in changing the transfer switch from the ONstate to the OFF state, and the following characteristic can beobtained.

With the above arrangement, in this embodiment, a satisfactorysolid-state image pickup element which has a residual image of 0.4% orless and is free from random noise in the dark state can be obtained.

In this embodiment, an NMOS transistor is used as the transfer switch,and NMOS transistors are used as MOS transistors of the gate drivecircuit. However, the effect of this embodiment can also be obtainedusing a PMOS transistor as the transfer switch and PMOS transistors asthe MOS transistors of the gate drive circuit.

FIG. 5 shows the gate drive circuit of a transfer switch according tothe fourth embodiment of the present invention. The same referencenumerals as in FIG. 4 denote the same members in FIG. 5. In thisembodiment, a constant current source Icont is used in place of the MOStransistors Q21 and Q31 in FIG. 4, as shown in FIG. 5.

In this embodiment, the device is designed such that the output from ascanning circuit for row selection goes high in the selection mode, andthe common control line of the gate of the transfer switch goes low whenturning on the transfer switch. As the transfer switch, an NMOStransistor is used.

This embodiment has the following characteristic features.

(1) The output unit includes the constant current source.

(2) The fall speed of the transfer switch is determined by thecapacitance of the gate line of the transfer switch and the currentvalue of the constant current source.

The device is designed to have a fall speed of 1.5 V/μsec. Since thefall speed is determined by the constant current source, the fall speedcab be accurately designed, and a structure resistant against avariation in power supply voltage can be obtained. As a result, asatisfactory solid-state image pickup element which has a residual imageof 0.1% or less and is free from random noise in the dark state can beobtained.

The fifth embodiment of the present invention will be described next.

In this embodiment, a gate drive circuit shown in FIG. 2 is used, andthe high-level, middle-level, and low-level voltages are set to 6 V, 2.5V, and 0 V, respectively. Referring to FIG. 12, the voltage of a powersupply line 102 is set to 3.5 V, thereby reducing the reset voltage ofthe floating diffusion area to 3.5 V. Consequently, the device is drivensuch that the potential structure shown in FIG. 17B is formed when asignal in the dark state is read out.

If the pocket shown in FIG. 17A is very large, the linearity on the lowilluminance side degrades. This is because the pocket is too large, andcharges in the low illuminance area, that are small in number, aretrapped by the pocket. When the high level of the transfer switch is setto be higher than the reset level of the floating diffusion area, as inthis embodiment, the pocket is filled with charges from the floatingdiffusion area, and the linearity on the low illuminance side improves.However, only with this measure, random noise abnormally increases dueto charges remaining in the pocket, resulting in large degradation inS/N ratio. According to the present embodiment, the random noise can bereduced by holding the middle level.

The sixth embodiment of the present invention will be described.

In this embodiment, a sensor block using the gate drive circuit of thetransfer switch of the first embodiment is used, thereby forming aone-chip solid-state image pickup apparatus with a 10-bit digitaloutput, which has a block structure shown in FIG. 6. When the presentinvention is used, a high-performance TG (timing generation block) 1501,PGA (programmable gain control amplifier) 1503, and ADC (AD converter)1504, which use a 0.35-μm general-purpose logic process, can be usedwhile maintaining the sensor performance. Hence, an inexpensive digitalsolid-state image pickup element with high performance can be provided.

In addition, a signal processing block 1605 for, e.g., image compressionshown in FIG. 7 can be added.

As described above, according to the first to sixth embodiments, thefollowing technological advantages can be obtained even in an APS usinga micropatterned MOS transistor.

(1) A satisfactory solid-state image pickup element almost free fromrandom noise or residual image can be provided.

(2) An inexpensive solid-state image pickup element that suppresses anydegradation in characteristics due to a process variation can beprovided.

(3) A one-chip solid-state image pickup element with an advancedfunction can be easily formed together with a high-performance logiccircuit or analog circuit using a micropatterned MOS transistor.

An embodiment wherein the solid-state image pickup element and a gatedrive circuit of any one of the above-described first to sixthembodiments is applied to a digital still camera will be described indetail with reference to FIG. 12.

Referring to FIG. 12, the camera has a barrier 1 serving as theprotection and main switch of a lens, a lens 2 for forming an opticalimage of an object onto a solid-state image pickup element 4, an iris 3for changing the amount of light transmitted through the lens 2, thesolid-state image pickup element 4 for receiving the object image formedby the lens 2 as an image signal, an A/D converter 6 for executinganalog-to-digital conversion of the image signal output from thesolid-state image pickup element 4, a signal processing unit 7 forexecuting various kinds of correction operations for the image dataoutput from the A/D converter 6 or compressing the data, a timinggeneration unit 8 (including a gate drive circuit) for outputtingvarious kinds of timing signals to the solid-state image pickup element4, image pickup signal processing circuit 5, A/D converter 6, and signalprocessing unit 7, a system control and operation unit 9 for executingvarious kinds of operations and controlling the entire digital stillcamera, a memory unit 10 for temporarily storing the image data, aninterface unit 11 for recording/reading out the image data on/from arecording medium, a detachable recording medium 12 such as asemiconductor memory for recording or reading out image data, and aninterface unit 13 for communication with an external computer or thelike.

The operation of the digital still camera with the above-describedarrangement in the phototaking mode will be described next.

When the barrier 1 is opened, the main power supply is turned on, thepower supply of the control system is turned on next, and finally, thepower supply of the image pickup system circuit such as the A/Dconverter 6 is turned on.

To control the exposure amount, the system control and operation unit 9sets the iris 3 in the full-aperture state. The signal output from thesolid-state image pickup element 4 is converted by the A/D converter 6and input to the signal processing unit 7.

The system control and operation unit 9 executes calculation forexposure on the basis of the data.

The brightness is determined on the basis of the result of photometry,and in accordance with the result, the system control and operation unit9 controls the iris.

On the basis of the signal output from the solid-state image pickupelement 4, a high-frequency component is extracted, and the distance tothe object is calculated by the system control and operation unit 9.After that, the lens is driven, and it is determined whether an in-focusstate is obtained. If it is determined that defocus state is obtained,the lens is driven again, and distance measurement is performed.

After confirming the in-focus state, actual exposure starts.

When exposure is ended, the image signal output from the solid-stateimage pickup element 4 is A/D-converted by the A/D converter 6, passesthrough the signal processing unit 7, and is written in the memory unitby the system control and operation unit 9.

After that, the data stored in the memory unit 10 is recorded on thedetachable recording medium 12 such as a semiconductor memory throughthe recording medium control I/F unit under the control of the systemcontrol and operation unit 9.

The image data may be directly input to a computer or the like throughthe external I/F unit 13 to process the image.

Many widely different embodiments of the present invention may beconstructed without departing from the spirit and scope of the presentinvention. It should be understood that the present invention is notlimited to the specific embodiments described in the specification,except as defined in the appended claims.

What is claimed is:
 1. A solid-state image pickup device comprising: aplurality of pixels each including a photoelectric conversion unit, afloating diffusion region, a transfer switch for transferring anelectric charge from the photoelectric conversion unit to the floatingdiffusion region, and an amplification transistor whose gate iselectrically connected to the floating diffusion region; and a scanningcircuit for supplying a drive pulse to a control node of the transferswitch, wherein the transfer switch is turned off by holding for apredetermined period a first level of the drive pulse supplied from thescanning circuit to the control node of the transfer switch, wherein thetransfer switch is turned on by holding for a predetermined period asecond level of the drive pulse supplied from the scanning circuit tothe control node of the transfer switch, and wherein a first period forchanging from the second level of the drive pulse to the first level ofthe drive pulse is longer than a second period for changing from thefirst level of the drive pulse to the second level of the drive pulse.2. The device according to claim 1, wherein the scanning circuit isconstructed to supply, to the transfer switch, a third level of thedrive pulse between the first and second levels of the drive pulse. 3.The device according to claim 2, wherein the third level is held for apredetermined period within the first period for changing from thesecond level to the first level.
 4. The device according to claim 1,wherein an initial speed of change from the second level to the firstlevel is larger than a speed of change from the first level to thesecond level.
 5. The device according to claim 1, wherein the firstperiod for changing from the second level to the first level is set tobe longer than the second period for changing from the first level tothe second level, to suppress the returning of an electric charge to thephotoelectric conversion unit.
 6. The device according to claim 1,wherein the photoelectric conversion unit includes a semiconductorregion accumulating the electric charge, and a portion of thesemiconductor region of the photoelectric conversion unit is arrangedunder a portion of a gate electrode of the transfer switch.
 7. Thedevice according to claim 1, wherein the transfer switch is a MOStransistor.
 8. The device according to claim 1, wherein each pixelfurther includes a reset switch, so that the photoelectric conversionunit is reset by turning on the reset switch during a period ofsupplying the second level of the drive pulse to the transfer switch. 9.A solid-state image pickup device comprising: a plurality of pixels eachincluding a photoelectric conversion unit, a floating diffusion region,a transfer switch for transferring an electric charge from thephotoelectric conversion unit to the floating diffusion region, and anamplification transistor whose gate is electrically connected to thefloating diffusion region; and a scanning circuit for supplying a drivepulse to a control node of the transfer switch, wherein the transferswitch is turned off by holding for a predetermined period a first levelof the drive pulse supplied from the scanning circuit to the controlnode of the transfer switch, wherein the transfer switch is turned on byholding for a predetermined period a second level of the drive pulsesupplied from the scanning circuit to the control node of the transferswitch, and wherein a speed of change from the second level of the drivepulse to the first level of the drive pulse is equal to or lower than10V/μsec.
 10. The device according to claim 9, wherein the scanningcircuit is constructed to supply, to the transfer switch, a third levelof the drive pulse between the first and second levels of the drivepulse.
 11. The device according to claim 9, wherein an initial speed ofchange from the second level to the first level is larger than a speedof change from the first level to the second level.
 12. The deviceaccording to claim 9, wherein the speed of change from the second levelto the first level is set to be equal to or lower than 10V/μsec, tosuppress the returning of an electric charge to the photoelectricconversion unit.
 13. The device according to claim 9, wherein thephotoelectric conversion unit includes a semiconductor regionaccumulating the electric charge, and a portion of the semiconductorregion of the photoelectric conversion unit is arranged under a portionof a gate electrode of the transfer switch.
 14. The device according toclaim 9, wherein the transfer switch is a MOS transistor.
 15. The deviceaccording to claim 9, wherein each pixel further includes a resetswitch, so that the photoelectric conversion unit is reset by turning onthe reset switch during a period of supplying the second level of thedrive pulse to the transfer switch.
 16. A solid-state image pickupdevice comprising: a plurality of pixels each including a photoelectricconversion unit, a floating diffusion region, a transfer switch fortransferring an electric charge from the photoelectric conversion unitto the floating diffusion region, and an amplification transistor whosegate is electrically connected to the floating diffusion region; and ascanning circuit for supplying a drive pulse to a control node of thetransfer switch, wherein a first period for changing from an ON state ofthe transfer switch to an OFF state of the transfer switch is longerthan a second period for changing from the OFF state of the transferswitch to the ON state of the transfer switch.
 17. The device accordingto claim 16, wherein the scanning circuit is constructed to supply, tothe transfer switch, a middle level between a first level correspondingto the ON state and a second level corresponding to the OFF state. 18.The device according to claim 17, wherein the middle level is held for apredetermined period within the first period.
 19. The device accordingto claim 16, wherein the scanning circuit is constructed to supply, tothe transfer switch, a first level corresponding to the ON state and asecond level corresponding to the OFF state, wherein an initial speed ofchange from the first level to the second level is larger than a speedof change from the second level to the first level.
 20. A solid-stateimage pickup device comprising: a plurality of pixels each including aphotoelectric conversion unit, a floating diffusion region, a transferswitch for transferring an electric charge from the photoelectricconversion unit to the floating diffusion region, and an amplificationtransistor whose gate is electrically connected to the floatingdiffusion region; and a scanning circuit for supplying a drive pulse toa control node of the transfer switch, wherein a speed of change from anON state of the transfer switch to an OFF state of the transfer switchis equal to or lower than 10V/μsec.
 21. A driving method of asolid-state image pickup device, wherein the solid-state image pickupdevice comprises a plurality of pixels each including a photoelectricconversion unit, a floating diffusion region, a transfer switch fortransferring an electric charge from the photoelectric conversion unitto the floating diffusion region, and an amplification transistor whosegate is electrically connected to the floating diffusion region; whereinthe method comprises driving the transfer switch from an OFF state to anON state and thence to the OFF state; wherein a first period forchanging from the ON state of the transfer switch to the OFF state ofthe transfer switch is longer than a second period for changing from theOFF state of the transfer switch to the ON state of the transfer switch.22. A driving method of a solid-state image pickup device, wherein thesolid-state image pickup device comprises a plurality of pixels eachincluding a photoelectric conversion unit, a floating diffusion region,a transfer switch for transferring an electric charge from thephotoelectric conversion unit to the floating diffusion region, and anamplification transistor whose gate is electrically connected to thefloating diffusion region; wherein the method comprises driving thetransfer switch from an OFF state to an ON state and thence to the OFFstate; wherein a changing speed from the ON state of the transfer switchto the OFF state of the transfer switch is equal to or lower than10V/μsec.